Experimental investigation of the role of specific cell surface alterations in mediating the altered growth behavior of virus-transformed cells is presently hindered by the general lack of experimental methods for manipulating the composition and organization of the cellular plasma membrane. Recent work in this laboratory has shown that lipid vesicles of defined composition can be fused with the plasma membrane of mammalian cells in vitro. Since lipid vesicles can be constructed from a wide range of lipids, and both glycolipids and glycoproteins can be incoporated within the walls of vesicles, the fusion of these structures with cultured cells offers a new experimental method for introducing a wide variety of components into the plasma membrane of living cells. Fusion of lipid vesicles composed of purified lipids of glycolipids with untransformed and virus-transformed cells will be used to induce alterations in plasma membrane lipid and glycolipid composition to study the effect of these changes on cell growth behavior and the expression of specific cell surface properties. Similar experiments are proposed using "membrane-vesicles" prepared from isolated purified plasma membranes. Fusion of "membrane-vesicles" with the cell surface will be used to introduce new plasma membrane components, including membrane proteins and glycoproteins, into the plasma membrane of different cell types. The use of both lipid vesicles and membrane-vesicles to induce defined alterations in plasma membrane composition in cultured cells will permit experimental analysis of the role of specific membrane components in cell growth regulation. These experiments will also provide informaton on how the changes in plasma membrane composition and cell surface organization found in virus-transformed cells are related to the altered growth behavior and abnormal surface properties displayed by these cells.